Electromagnetic interference (EMI) can be characterized as undesirable voltages and currents that emanate from one electrical or electronic device and alter or degrade the performance of other electrical and electronic devices that are either proximate or connected to the first device. EMI emissions may transmitted either by conduction through electrical power lines or by radiation through space.
Electric motors such as those used in various actuator assemblies are a significant source of EMI in automotive vehicles. Most EMI emissions from electric motors originate from at least one of two primary sources. One of these sources is electrical arcing that typically occurs between the brush and commutator portions of the motor. EMI emissions that result from electrical arcing are radiated outward from the source and can reach sufficiently far from the source to affect electronic devices in the vehicle--even when there is no common current path between the motor and the affected devices. The other primary EMI source in an electric motor is the interruption of current flow that occurs in the motor commutator as an armature portion of the motor rotates. EMI produced by this source travels to electronic devices in the vehicle by conduction through electrical leads extending from the motor and connected to the vehicle electrical system.
General Motors (GM) has established EMI limits that automotive electric motors must not exceed if they are to avoid degrading electronic devices in the vehicle. To reduce EMI emissions from electric motors to a level below the established limit, it is necessary to reduce the levels of both radiated and conducted EMI that the motors produce.
One method that is known to help minimize EMI conduction is to provide a filter between the EMI source and the affected devices. Such filters are configured to reduce the level of EMI conducted to the affected devices. A method known to reduce EMI radiation is the use of a ground shield that surrounds the EMI source and blocks radiated EMI emissions.
One example of a particularly problematic EMI source is a known rear compartment deck lid actuator assembly that has a very compact configuration and is powered by an electric motor. As shown in FIG. 10, without any EMI reduction, EMI emissions from this known assembly exceed, by a considerable margin, EMI limits established by the manufacturer of the automobile that the actuator is installed in. The manufacturer's EMI limits are represented by line 18.
Due to part integration limitations, it would be undesirable to make actuator assemblies such as the known assembly discussed above larger to accommodate EMI-reducing hardware or to include an EMI-reducing add-on component attached to the side of a plastic housing portion of the actuator assembly.
What is needed is a cost effective method and apparatus for reducing EMI emissions from a rear compartment deck lid actuator assembly.